(a) Technical Field
The present invention relates to a metal separator for a fuel cell and to a method of treating the surface of the metal separator to form same. More particularly, the present invention relates to a metal separator for a fuel cell whereby the surface of the metal separator is treated to have improved corrosion-resistance, reduced contact resistance and improved electrical conductivity as compared to a conventional fuel cell metal separator.
(b) Background Art
In the area of fuel cell technology, a current aim is to develop a metal separator for use in a fuel cell which can replace existing graphite separators and which has a surface structure that is capable of overcoming the poor corrosion resistance typically seen with existing metal separators, as well as displaying improved stable electrical conductivity and other improved properties. Fuel cell stack separators account for more than 50% of their overall cost. Metal separators will help reduce these costs, however, certain hurdles as to their use must first be overcome.
To date, a variety of metal materials, including steel, stainless steel, and aluminum, etc., have been studied for their application as fuel cell separators. Among them, stainless steel is typically used most often as a metal separator for fuel cells.
Conducting the proper surface treatment of a metal separator for a fuel cell is important in determining the overall characteristics of the separator. Various methods for carrying out surface treatments of a metal separator are known. For example, physical vapor deposition processes whereby the surface is coated with carbide or nitride to form a hardened coating layer (e.g., CrN or TiN coating layer) can be used. Also, surfaces can be treated by a plasma nitriding process (i.e., a heat treating process that alloys nitrogen onto the surface of a metal to create a hardened surface) by which a nitride layer is formed on the surface of a metal using a plasma deposition process that can be operated at a temperature lower than 600° C.
The high-quality coating layers, such as CrN or TiN, produced by physical vapor deposition processes usually provide superior corrosion resistance which satisfies the requisite level of corrosion resistance of a fuel cell separator. However, such methods produce surfaces having relatively high contact resistance, and also have high production costs due to the high vacuum conditions. Accordingly, such methods are impractical with regard to mass production of such materials.
Problems are also associated with preparing such materials by plasma nitriding. While a surface nitride layer produced by plasma nitriding provides good cost competitiveness and mass producibility, such processes, through interdiffusion of chromium and nitrogen, result in extensive pore formation near the surface of the material. In addition, nitride is formed from bonding with the bulk chromium. As a result, a large amount of chromium is consumed in the process, which results in the formation of a chromium-deficient layer, thereby greatly decreasing corrosion resistance of the material.
Thus, oxidation and corrosion may occur at the surface of such materials if prepared using the above existing techniques. Moreover, although corrosion resistance is improved where a thick oxide layer is formed on the surface, the surface contact resistance at the same time rises rapidly, making the treated metal inoperable as a separator for use in a fuel cell.
To solve these problems, research has focused on developing materials for forming fuel cell separators that contain lower amounts of chromium, e.g., 25%. However, while corrosion resistance may be improved with less chromium, processability is degraded if the chromium content exceeds 20%. Thus, it is difficult to form flow channels for hydrogen, oxygen, water, etc. with desired shape through press processing because of excessive springback. Improved materials and methods which can be used to produce viable metal separators for fuel cells as realistic substitutes for graphite separators would be an advance in the art.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention. Therefore the above may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.